Integrated reflectance photometric sorter

ABSTRACT

Apparatus for sorting objects from a stream of objects moving through a sorting zone in accordance with light reflected from the objects, has a light source to direct light into the sorting zone across the width of the stream and a single line array of photodiodes each having a photosensitive semiconductor junction adapted for operation in a photon flux storage mode. An electronic switch arrangement which scans the photodiodes by connecting the output of each photodiode singly in and in succession through the array to a common output. The signal at the common output represents a succession of values of summed or integrated reflected light which impinged on each photodiode during a predetermined interval. The predetermined interval being the time period between a particular photodiode being connected to the common output on successive switching scans or sweeps. A plurality of deflection devices, such as air nozzles, is in the sorting zone downstream of the photodiodes and extending across the width of the path in side by side relationship so that each deflection device defines a channel. Timing means is provided to (a) cycle the switching arrangement and (b) provide a timing signal relating the signal at the common output to respective channnels. A control circuit is connected to receive the timing signals and the signal at the common output to actuate the deflection devices to deflect objects which reflected light of predetermined characteristics.

United States Patent I 91 Kelly 1 INTEGRATED REFLECTANCE PHOTOMETRICSORTER [75] Inventor: Leonard Kelly, Peterborough,

Canada [73] Assignee: Sphere Investments Limited,

Nassau, Bahamas [22] Filed: Mar. 22, 1974 [21] Appl. No.: 453,626

[30] Foreign Application Priority Data Mar. 22, 1973 United Kingdom 13892/73 [52] US. Cl. 209/l l1.7; 250/209; 356/209 51 Int. Cl. ..B07C 5/342[58] Field of Search 209/111.5, 111.6, 111.7; 7 356/209; 250/578, 209

[56] References Cited UNITED STATES PATENTS I 3,545,610 12/1970 Kelly eta l 209/11 1.7 X 3,822,362 7/1974 Weckler et al. 250/578 X FOREIGNPATENTS OR APPLICATIONS 5/1953 United Kingdom 209/1 1 1.7

Primary ExaminerRobert B. Reeves Assistant Examiner-Joseph J. RollaAttorney, Agent, or FirmAlex. E.;MacRae & Co.

CLOCK- GENEZATOKJ [451 Aug. 26, 1975 [57] ABSTRACT Apparatus for sortingobjects from a stream of objects moving through a sorting zone inaccordance with light reflected from the objects, .has a light source todirect light into the sorting zone across the width of the stream and asingle line array of photodiodes each having a photosensitivesemiconductor junction adapted for operation in a photon flux storagemode. An electronic switch arrangement which scans the photodiodes byconnecting the output-of each photodiode singly in and in successionthrough the array to a common output. The signal at the common outputrepresents a succession of values of summed or inte- I grated reflectedlight which impinged on each photodiode during a predetermined interval.The predetermined interval being the time period between a particularphotodiode being connected to the common output on successive switchingscans or sweeps. A plurality of deflection devices, such as air nozzles,is in the sorting zone downstreamfof the photodiodes and extendingacross the width of the path in side by side relationship so that eachdeflection device defines a' channel. Timing means is provided to (a)cycle the switching arrangement and (b) provide a timing signal relatingthe signal at the common output to respective channnels. A controlcircuit is connected to receive. a the timing signals and the signal atthe common output to actuate the deflection devices to deflect objectswhich reflected light of predetermined characteristics.

1 Claim, 1 rawin Figure D LECTIOH EF CONTWL DCCIQION AND TIMING E I 2INTEGRATED REFLECTANCE PHOTOMETRIC SORTER BACKGROUND OF THE INVENTIONThis invention relates to an apparatus for sorting objects according totheir radiation reflectance properties, and in particular according totheir light radiation reflectance properties where the light radiationis intended to include infra red radiation and ultra violet radiation. 4

The invention is particularly adapted to the sorting of pieces orfragments of ore and will be described in connection with the sorting ofore. However, it will be realized that the invention is suitable for thesorting of other objects.

There are a number of known devices for sorting or sizing pieces of orephotometrically, and they can be classified generally into three groupsaccording to the type of light detector used. The three groups are: (1)devices which use a photomultiplier as the light detector, (2) deviceswhich use as the light detector a junction semiconductor operating in aninstantaneous photoconductive or photovoltaic mode, and (3) deviceswhich use a television camera tube as the light detector.

Devices falling within the first group, which use photomultipliers,generally use a lens system and a field stop to define the field ofview. If scanning is used it may be accomplished by various mechanicalmeans. For example, it may be accomplished by sweeping the field stopusing a Nipkow Disc, or by sweeping the reflected or incoming radiationusing a mirror drum, or by sweeping the source of radiation such as alaser beam. Occasionally both the radiation source and the incomingradiation may be swept in synchronism, as for example as described in USPat. No. 3,545,6l to Leonard Kelly et al., issued Dec. 8, 1970.

Photomultipliers achieve good sensitivity at low light levels because oftheir large photocathodes and internal amplification, but they have thefollowing disadvantages. (a) Photomultipliers respond to instantaneouslight levels only; there is virtually no storage or photo fluxintegration involved. (b) Photomultipliers have an extremely small photocurrent and they require great amplification in the secondary emissionmultiplier stage. Noise and the statistical fluctuations of photoemission and secondary emission limit the accuracy of response. (0)Scanning must be done externally and mechanically and this entailsvibration, wear, limited life, and also requires a speed control orsynch control. (d) Fatique, susceptibility to mechanical damage, therequirement for a high-voltage power supply of excellent stability,cost, accuracy, and variation from unit to unit are other disadvantages.

Devices falling within the second group use as a light detector thejunction of a semiconductor operating in an instantaneousphotoconductive or photovoltaic mode. In this mode the output isproportional to incident light radiation at any given instant. Generallyseveral lensed photodiodes or phototransistors are arranged in a row andare pointed directly at a light source. Objects interposed between thelight source and the detectors cause output signals, of a single binarynature, related to size; plan area or profile only. The disadvantages orlimitations of semiconductors used in this instantaneous photon fluxmode are as follows: (a) The active area is very small. When operated inthis instantaneous mode, they require a high intensity of radiation toproduce a useful output signal above the noise in the photodetector andamplifier stages. This precludes their use as detectors of reflectedradiation. In practice a strong light source is beamed directly at thedetectors and interruption of this radiation produces a signal which isessentially binary in nature, rather than analog. (b) The resolution ofan array for determining size is governed by the number and spacing ofindividual detectors. Since each detector requires its own circuit and aseparate connection to a logic gate, the practical difficulties ofmounting, spacing and interconnection limit the scan resolution of thesesystems. i i

Devices falling within the third group, using television camera tubes,usually use either a Vidicon or Plumbicon type of tube. The greatadvantage of using a television camera type of detector is that thesedetectors make use of the storage principle to produce useable signalsfrom relatively low brightness image characteristics. This type ofdetector is therefore suitable for sorters operating on reflected light.In the storage principle the elements of a photosensitive surface arescanned by an electron beam once every scan interval. In between scansthe local photon flux is integrated. It is this integration whichprovides the useable signal. The television camera system has severaldisadvantages and limitations which are indicated in the followingmaterial. (a) The persistence or lag caused by the time constant of thephotoconductive process is a definite limitation. For example, in atypical Vidicon tube it may take 50 milliseconds for the photocurrent todecay to 20 of its original value after illumination is removed. Amodern sorting apparatus may handle pieces of rock travelling in excessof 1,000 ft/minute (5 milliseconds/inch). It is unacceptable to have theimage of a rock still contributing to the signal being produced by thescan when the rock is in fact 10 inches downstream of the scan zone. (b)Non-linearity and noise are masked by phosphor characteristics and eyeaccommodation when the video signal is used to reproduce a display on amonitor, however, when the video signal is used as an input to acomparator in a sorting application, non-linearity and noise aresignificant sources of error. (e) Vidicon type image pick-up tubes aresusceptible to light burn of the photoconductor, electron beam rasterburn, thermionic cathode failure and vacuum leaks. Also, degradationoccurs during what is normally a relatively short life.

SUMMARY OF THE INVENTION It is an object of the present invention toreduce or overcome disadvantages in the three groups of sorting devicesreferred to above and to provide a novel sorting apparatus.

The present invention makes use of a closely space single line array ofphotodiodes operating in a photon flux integrating mode as a selfscanning light detector. This allows low values of reflected light to bedetected, with high resolution, and is consequently unlike theinstantaneous value discrete photodiode systems mentioned above. Thescan is electronic and not mechanical which makes it capable of veryhigh scan rates. In addition noise is low, lag is negligible, theassociated drive and logic is simple, and the response is linear withgreat dynamic range.

In the present invention, which uses multiple deflection channels, thephotodetector scan timing, circuit is also used to synchronize thedemultiplexing of the video data.

The present invention provides an apparatus for sorting in accordancewith light reflected from a stream of objects moving through a sortingzone along a predetermined path having a predetermined width, comprisinga single line array of photodiodes each having a photosensitivesemiconductor junction and adapted for operation in a photon fluxstorage mode, illuminating means to direct source of light into saidsorting zone across said predetermined width of said path, a lens systemto focus an image of said stream extending across the illuminated widthof said path onto said array, a plurality of deflection means in saidsorting zone extending across the width of said path in side by siderelationship, each deflection means defining a channel, switch means forconnecting to a common output each photodiode singly and in successionthrough said array to provide at said common output a first signalrepresenting a succession of values of reflected light impinging on eachphotodiode during a predetermined interval, timing means to cycle saidswitch means successive operations through said array and to providetiming signals relating portions of said first signal to respectivechannels, and control means responsive to said first signal and to saidtiming signals to cause actuation of respective deflection means todeflect objects which reflected light of predetermined characteristics.

BRIEF'DESCRIPTION OF THE DRAWING The single FIGURE is a drawing, partlyin schematic block diagram form, showing the invention.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawing there is showna belt passing over a head pulley 11. The belt 10 carries a stream ofpieces of rock 12 distributed on it to form a limited width stream ofrocks in a single layer. Only a portion of belt 10 is shown as it iswell known in the art to have a sorting apparatus with a moving beltwhich carries a single layer of objects on it. One example of such asystem is described in United States patent application No. 391,909 toWilhelm Hieronymus, filed Aug. 27, 1973 and another inaforementionedU.S. Pat. No. 3,545,610. The pieces of rock 12 are discharged from thebelt 10 in a trajectory which passes in front of a deflection device 14which comprises a plurality of nozzles 15 each with a separate supply offluid connected to it. The fluid is normally compressed air andthissupply is indicated by arrows and letters a j shown beneath deflectiondevice 15. Such deflection devices are also well known and aredescribed, for example in the aforementioned U.S. Pat. No. 3,545,610.The nozzles 15 may be considered as dividing the stream into multipledeflection channels.

A light source (not shown) illuminates the pieces of rock in thescanning zone. A lens system, indicated schematically at 16, focuses animage of the stream of rocks onto a scanning means 17. The broken lines18 indicate this. The broken lines 18 indicate the scanning zone on belt10.

The scanning means 17 may comprise a single line array and preferably alinear array of photodiodes 20 with the axis of the array beingtransverse to the flow of stream of rocks. The photodiodes arepreferably integrated into a single monolithic chip and may have asensitive area of approximately l mil and be spaced on 1 mil centres.Thus, a sensitive area of 0.5 inch X 0.001 inch would contain 500photodiodes, and the corresponding image would have 500 elements. Thisis, if a 25 inch scan were used there would be 500picture elements onthe scanned area, each being 0.050 inch 0.050 inch. Photodiodes arrangedin monolithic chips of this nature are available on the market.

The scanning means 17 of photodiodes 20 has associated with it acorresponding array of switches 21 which are preferably FET typeswitches. The switches 21 serve to connect each photodiode 20 to acommon video output line 22. That is, when an individual switch 21 isclosed it connects the associated photodiode 20 to output line 22. Ashift register 23 generates a sequence of signals or signal pulses whichactuates each photodiode 20 singly and in turn. This provides theelectronic scan. It is convenient to have the array of photodiodes 20,the switches 21 and the shift register 23 integrated on a single chip.Such an integrated device requires only power and output connections anda drive for the shift register. There are integrated devices of thistype available on the market.

A simplified circuitry and apparatus for adapting a linear array ofphotodiodes, as described, to a sorting operation will now be described.The drive for shift register 23 may comprise a clock generator 24 forproviding the clock pulses for operation, a presettable counter 25, anda start pulse generator 26. The counter 25 counts the clock pulses up tothe number of photodiode elements in the array, then the counter 25resets and provides a signal to the start pulse generator 26 causing itto generate a start pulse. The start pulse is applied to shift register23 to initiate the shift register sequence, and the clock pulses fromgenerator 24 are also applied to shift register 23 to cause operation ofthe shift register sequence once it is initiated.

The start pulses from pulse generator 26 is also applied to adivider/counter 27, and the clock pulses from clock generator 24 arealso applied to the divider/counter 27. The divider 27 gives an outputpulse after it receives a predetermined number of clock pulses. Thepredetermined number is chosen to correspond to the number ofphotodiodes which view a single one of the multiple channels. That is,the divider 27 provides one output pulse as the scan crosses each one ofthe multiple channels. This output pulse from divider 27 is applied toconverter 28.

Converter 28 may not be required in some circuit arrangements as will beapparent to those skilled in the art. In a preferred form converter 28is a form of counter which counts the pulses representing the passage ofa scan across each channel and produces a binary coded digital signalrepresenting the number on parallel output lines. The output of counter28 or converter 28 is applied to demultiplexer 30 which decodes thebinary information to sequentially gate open individual channels. Asignal representing video information is also applied to demultiplexer30 over conductor 31. Thus, the demultiplexer 30 serves to switch theincoming signal representing the video information to the outputs,indicated as a j, in sequence, so that the video informationrepresenting the scan in the first channel or the achannel goes to theoutput a of the demultiplexer. Circuitry for demultiplexing in thismanner is well known.

As was previously mentioned, the video or scan information is availableon the conductor or common video output line 22, and this is in the formof pulses of varying amplitude. A circuit represented by a power supply33 and a resistance 34 is connected to line 32. Each photodiode isreverse biased by power supply 33 through resistance 34 and line 22 asits respedtive one of switches 22 is turned on and is then opencircuited for the remainder of the scan period when the switch is ofF orin the high resistance state. If light falls on the photodiode duringthe time when the switch is in its off condition or open circuited, aphoto-generated current discharges the junction capacitance and theamount of charge removed is proportional to the total incidentillumination in that interval of time. When the switch is momentarilyswitched to its on condition, the photodiode is recharged throughresistance 34 and line 22. The charging current pulse appears acrossload resistance 34 as a pulse of video information. Thus the videoinformation which consists of a repeated sequence of these pulses, isapplied to amplifier 32. The amplified video signal may be passedthrough a video processor 35 to conductor 31 and the demultiplexer 30.The video processor 35 may convert the basic analog video data to binaryform if this is required in subsequent stages. The video processor 35may include transient blanking gates to eliminate any transients whichappear as a result of the switching. The video processor may includeautomatic gain control circuitry, or it may include any other desiredcircuitry.

The output from the demultiplexer 30 appears on individual channelcircuits as indicated in the drawing as a j. The individual channelcircuits are identical and only one (associated with channel a) will bedescribed. The video output for channel a is applied to a decision andtiming circuit 36. This circuit compares the video signal it receives insome manner to a predetermined or derived reference or references andprovides an output if deflection is to take place. The decision andtiming circuit may be of the type described in the aforementioned US.Pat. No. 3,545,610 although other types may be used. The output fromdecision and timing circuit 36 is applied to a deflection control device37 which normally includes a valve to open and close between a supply offluid under pressure and the nozzle at a in the deflection device 15.

It is a feature of the present invention that a single master oscillatoror clock pulse'generator 24 is used to synchronize both the scanningmeans and the demultiplexer. This simplifies the apparatus whilemaintaining a high degree of precision. For example, the frequency ofoperation of clock pulse generator 24 may be adjusted to meet variousrequirements without affecting the precision. The signal for eachphotodiode remains locked to the correct channel. If the clock frequencyis reduced, a lower scan frequency will result with a proportionalincrease in video signal for a given light level. This is because of theintegrating ability of the photodiode in the present arrangement.However the reduced frequency does not result in any adjustment orsynchronization problems. In addition, it is a relatively simple matterto change the number of photodiodes representing a channel, or thenumber of channels per scan, as may be required for a particular sorterconfiguration.

The linearity of the scan division is precise. This precision derivesfrom two features which provide improved results over an electron beamtype of scan or a mechanical scan. First, since a plane surface (thestream of objects on a belt) is being imaged onto a line of photodiodesin a flat plane, equal divisions at the object plane appear as equaldivisions on the image plane, regardless of position with respect to theaxis of the lens system. Thus, the resolution does not vary from onepart of the scan to another. Second, the clock generator is used both asa timer and a switching control. While the timing is important, thelocking together of the scanner switching and the demultiplexing by adirect count rather than by time contributes to the accuracy of thescan. This count/scan position relationship results in much simplerchannel gating and video processing as compared to an electron beam scan(Vidicon type) or a mechanical scan. Both the electron beam scan and themechanical scan use time as a basis for scan position and channeldivision and linearity is frequently a problem.

I claim:

1. Apparatus for sorting in accordance with light reflected from astream of objects moving through a sorting zone along a predeterminedpath having a predetermined width, comprising a single line arraycomprising a plurality of photodiodes integrated into a singlemonolithic chip, each having a photosensitive semiconductor junction andadapted for operation in a photon flux storage mode,

illuminating means to direct a source of light into said sorting zoneacross said predetermined width of said path,

a lens system to focus an image of said stream extending across theilluminated width of said path onto said array, plurality of deflectionmeans in said sorting zone extending across the width of said path inside by side relationship, each deflection means defining a channel, thenumber of photodiodes being of an order of magnitude greater than thenumber of channels, switch means compressing a semiconductor switch foreach photodiode for connecting to a common output each photodiode singlyand in succession through said array to provide at said common output afirst signal representing a succession of values of reflected lightimpinging on each photodiode during a predetermined interval,

timing means comprising a shift register having a unit for each saidsemiconductor switch, a source of clock pulses to step said shiftregister through successive cycles to actuate said semiconductorswitches in sequence, and a counter to count a predetermined number ofclock pulses corresponding to a channel and to provide a timing signalfor each count relating portions of said first signal to respectivechannels, and

control means including a demultiplexer having a section for eachchannel, each section being connected for actuation of a respectivedeflection means, said sections being enabled in sequence by said timingsignal from said counter, and a signal treating means for processingsaid first signal and applying the processed signal to saiddemultiplexer for actuating enabled deflection means for the deflectionof objects which reflected light of predetermined characteristics.

1. Apparatus for sorting in accordance with light reflected from astream of objects moving through a sorting zone along a predeterminedpath having a predetermined width, comprising a single line arraycomprising a plurality of photodiodes integrated into a singlemonolithic chip, each having a photosensitive semiconductor junction andadapted for operation in a photon flux storage mode, illuminating meansto direct a source of light into said sorting zone across saidpredetermined width of said path, a lens system to focus an image ofsaid stream extending across the illuminated width of said path ontosaid array, a plurality of deflection means in said sorting zoneextending across the width of said path in side by side relationship,each deflection means defining a channel, the number of photodiodesbeing of an order of magnitude greater than the number of channels,switch means compressing a semiconductor switch for each photodiode forconnecting to a common output each photodiode singly and in successionthrough said array to provide at said common output a first signalrepresenting a succession of values of reflected light impinging on eachphotodiode during a predetermined interval, timing means comprising ashift register having a unit for each said semiconductor switch, asource of clock pulses to step said shift register through successivecycles to actuate said semiconductor switches in sequence, and a counterto count a predetermined number of clock pulses corresponding to achannel and to provide a timing signal for each count relating portionsof said first signal to respective channels, and control means includinga demultiplexer having a section for each channel, each section beingconnected for actuation of a respective deflection means, said sectionsbeing enabled in sequence by said timing signal from said counter, and asignal treating means for processing said first signal and applying theprocessed signal to said demultiplexer for actuating enabled deflectionmeans for the deflection of objects which reflected light ofpredetermined characteristics.